Re: Transformer drawing 1 amp

On 14 Mar 2007 05:17:55 -0700, Bob.Jones5400@xxxxxxxxx wrote:

I have a transformer that is drawing about 1 amp when the secondary is
not in use. Is this normal? Its a 15amp 12V CT transformer. The
secondaries read 12 volts from the center like it should(its actually
about 13V). It had what I guess what a thermistor(a rectangular box
that as stuck inside the transformers windings) but I took it out
because I thought it might be the problem.

What are the dimensions of the transformer core? I happen to have a
transformer rated 8 amps @ 24 volts, about the same rating as yours. It's wound
on a 2" stack of EI-137 laminatinos:
http://www.laminationspecialties.com/EI/EI137.htm

A really good way to detect shorted turns is to measure the unloaded loss of
the transformer in question. This requires a wattmeter, which you probably
don't have, but just to show some typical measurements, I slipped a single turn
of 21 gauge wire around the center leg of my transformer, and also a single turn
of 14 gauge wire.

I then measured the current with 120 VAC applied, and with all secondaries
open and no shorted turns. Then I successively shorted the single turn of 21
gauge wire and the single turn of 14 gauge wire.

The results were as follows:

Primary current Wattmeter reading

No shorts .187 A 10.5 watts

21 ga shorted .280 A 25.0 watts

14 ga shorted .65 A 67.0 watts

With only 10.5 watts dissipated, the transformer should only get a little warm
after an hour or so. With 67 watts dissipated, I would expect it to get hot.
The description you give in another post sounds like it's hotter than it should
be with no load.

If it's about the same size as my exemplar, then the primary current of .6
amps with no load, plus the temperature rise you are seeing, would seem to
indicate a shorted turn.

I also did a short circuit test by placing an ammeter across the secondary and
bringing up the primary voltage slowly with a variac. With 8 amps (rated
current) in the ammeter connected to the secondary, the wattmeter indicated 10
watts dissipation. This means that at rated load, the copper loss in the
transformer is 10 watts, and with a core loss of slightly less than 10.5 watts,
the total loss (core + copper) is about 20 watts at full load, substantially
less than the 67 watts shown above. If this transformer is similar to yours,
and it were operated at full load, you would have more loss due to a shorted
turn than due to the load.

The insulation system is probably not designed for this much temperature rise,
especially in the immediate vicinity of the shorted turn, The transformer would
smell and more shorted turns would eventually result. :-( Without the
protective overtemp cutout you removed, there would be danger of a fire.


It hums like crazy when I connect power to it and everything seems to
work fine with it except that its drawing 1 amp for no reason. I
seriously doubt this is power loss in the transformer but I can't
figure out whats wrong. The only thing I can think of is that a few
windings might have fused together somewhere but surely this would
cause a huge current draw?

The transformer doesn't look like there is anything wrong with it(no
charred spots) and it came from a car battery charger. I'm trying to
turn it into a variable power supply (about 5 to 12V) but I just have
no clue if I should chunk the transformer or not. I essentially have
nothing connected to the transformer except a switch that works and
the power cord that also works(they are not the issue here). I
connected an amp meter between the switch and one of the transformer
leads and its reading about .8A AC after start up(there is a surge at
the start but it seems to settle about there).


------ SWITCH ------- ----- AMP meter ----- --- +

Transformer
------ Neutral ------- ----------------------------------- +

Anyone have any clues on whats going on or how I could farther
diagnose the issue? Or is this natural for transformers to waste so
much energy? (I thought they were pretty efficient and being that I
have nothing connected on the secondary I would expect it not to waste
any power except maybe losses in the core)

Thanks,
Bob

.

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